diff --git a/nova/auth/signer.py b/nova/auth/signer.py
index c0468021b..accc808b3 100644
--- a/nova/auth/signer.py
+++ b/nova/auth/signer.py
@@ -53,8 +53,8 @@ import boto
 # NOTE(vish): for old boto
 import boto.utils
 
+from nova import exception
 from nova import log as logging
-from nova.exception import Error
 
 
 LOG = logging.getLogger('nova.signer')
@@ -81,8 +81,7 @@ class Signer(object):
     def generate(self, params, verb, server_string, path):
         """Generate auth string according to what SignatureVersion is given.
 
-        The signature method defaults to SHA256 if available, or falls back to
-        SHA1 if not.
+        The signature method must be SHA1 or SHA256.
 
         """
         if params['SignatureVersion'] == '0':
@@ -91,7 +90,7 @@ class Signer(object):
             return self._calc_signature_1(params)
         if params['SignatureVersion'] == '2':
             return self._calc_signature_2(params, verb, server_string, path)
-        raise Error('Unknown Signature Version: %s' %
+        raise exception.Error('Unknown Signature Version: %s' %
                     params['SignatureVersion'])
 
     @staticmethod
@@ -132,12 +131,20 @@ class Signer(object):
         """Generate AWS signature version 2 string."""
         LOG.debug('using _calc_signature_2')
         string_to_sign = '%s\n%s\n%s\n' % (verb, server_string, path)
-        if self.hmac_256:
+
+        if 'SignatureMethod' not in params:
+            raise exception.Error('No SignatureMethod specified')
+
+        if params['SignatureMethod'] == 'HmacSHA256':
+            if not self.hmac_256:
+                raise exception.Error('SHA256 not supported on this server')
             current_hmac = self.hmac_256
-            params['SignatureMethod'] = 'HmacSHA256'
-        else:
+        elif params['SignatureMethod'] == 'HmacSHA1':
             current_hmac = self.hmac
-            params['SignatureMethod'] = 'HmacSHA1'
+        else:
+            raise exception.Error('SignatureMethod %s not supported'
+                                  % params['SignatureMethod'])
+
         keys = params.keys()
         keys.sort()
         pairs = []
@@ -157,5 +164,6 @@ class Signer(object):
 
 
 if __name__ == '__main__':
-    print Signer('foo').generate({'SignatureVersion': '2'}, 'get', 'server',
-                                  '/foo')
+    print Signer('foo').generate({'SignatureVersion': '2',
+                                  'SignatureMethod': 'HmacSHA256'},
+                                  'get', 'server', '/foo')
diff --git a/nova/tests/test_signer.py b/nova/tests/test_signer.py
index 8a223f60d..80573e4a8 100644
--- a/nova/tests/test_signer.py
+++ b/nova/tests/test_signer.py
@@ -54,14 +54,33 @@ class SignerTestCase(test.TestCase):
                                 'PUT',
                                 '/johnsmith/photos/puppy.jpg'))
 
-    def test_generate_using_version_2(self):
+    def test_generate_HmacSHA256(self):
         self.assertEquals('clXalhbLZXxEuI32OoX+OeXsN6Mr2q4jzGyIDAr4RZg=',
                           self.signer.generate(
                                 {'SignatureMethod': 'HmacSHA256',
                                  'SignatureVersion': '2'},
                                 'GET', 'server', '/foo'))
 
-    def test_generate_force_HmacSHA1(self):
+    def test_generate_HmacSHA1(self):
+        self.assertEquals('uJTByiDIcgB65STrS5i2egQgd+U=',
+                          self.signer.generate({'SignatureVersion': '2',
+                                           'SignatureMethod': 'HmacSHA1'},
+                                           'GET', 'server', '/foo'))
+
+    def test_generate_invalid_signature_method_defined(self):
+        self.assertRaises(exception.Error,
+                          self.signer.generate,
+                          {'SignatureVersion': '2',
+                           'SignatureMethod': 'invalid_method'},
+                          'GET', 'server', '/foo')
+
+    def test_generate_no_signature_method_defined(self):
+        self.assertRaises(exception.Error,
+                          self.signer.generate,
+                          {'SignatureVersion': '2'},
+                          'GET', 'server', '/foo')
+
+    def test_generate_HmacSHA256_missing_hashlib_sha256(self):
         # Stub out haslib.sha256
         import hashlib
         self.stubs.Set(hashlib, 'sha256', None)
@@ -69,20 +88,24 @@ class SignerTestCase(test.TestCase):
         # Create Signer again now that hashlib.sha256 is None
         self.signer = signer.Signer(
                         'uV3F3YluFJax1cknvbcGwgjvx4QpvB+leU8dUj2o')
-        self.assertEquals('uJTByiDIcgB65STrS5i2egQgd+U=',
-                          self.signer.generate({'SignatureVersion': '2'},
-                                               'GET', 'server', '/foo'))
+        self.assertRaises(exception.Error,
+                          self.signer.generate,
+                          {'SignatureVersion': '2',
+                           'SignatureMethod': 'HmacSHA256'},
+                          'GET', 'server', '/foo')
 
     def test_generate_with_unicode_param(self):
         self.assertEquals('clXalhbLZXxEuI32OoX+OeXsN6Mr2q4jzGyIDAr4RZg=',
-                          self.signer.generate({'SignatureVersion': u'2'},
-                                               'GET', 'server', '/foo'))
+                          self.signer.generate({'SignatureVersion': u'2',
+                                            'SignatureMethod': 'HmacSHA256'},
+                                            'GET', 'server', '/foo'))
 
     def test_generate_with_non_string_or_unicode_param(self):
         self.assertEquals('99IAgCkhTR2aMTgRobnzKGuNxVFSdb7vlQRvnj3Urqk=',
                           self.signer.generate(
                               {'AnotherParam': ClassWithStrRepr(),
-                               'SignatureVersion': '2'},
+                               'SignatureVersion': '2',
+                               'SignatureMethod': 'HmacSHA256'},
                               'GET', 'server', '/foo'))
 
     def test_generate_unknown_version(self):